The present invention relates to an image recording apparatus capable of recording a halftone image with high resolution.
A systematic dither method is an implementation available with a digital copier or similar image recording apparatus for reproducing an image having shades or halftone image. With a systematic dither method, however, it is difficult to reproduce image portions where the density sharply changes. For example, such a dither method blurs the edges of a two-level image such as a character or a line. Moreover, when a halftone image or dot image on a printing or similar matter is reproduced by the above-mentioned method, the periodic pattern of the dots and the dither pattern interfere with each other to cause moire to appear. An error scattering method (average error minizing method) is another conventional method which has relatively high resolution, as disclosed in Japanese patent laid-open publication No. 5677/1986, for example. The error scattering method is such that a threshold for a pixel of interest included in an input image is determined in consideration of the levels of surrounding pixels. Generally, the systematic dither method promotes easy control over tone characteristics, but it is not satisfactory in resolution. On the other hand, the error scattering method is excellent in resolution, but it cannot readily control tone characteristics. In the light of this, there has been proposed an image processing method which determines whether an original image is a photographic image portion with continous tones (tone image area) or a character, line or similar image (two-level image) and, depending on the result of decision, selects either one of the systematic dither method or the error scattering method. This kind of method is taught in, for example, Japanese patent laid-open publication Nos. 19872/1989 and 19873/1989.
Recently, a laser printer capable of recording a dot with any of several density levels has been reported, which is a remarkable improvement over the traditional black-and-white printing. Dither patterns for implementing the systematic dither method have been developed in relation to such an advanced laser printer. However, among the problems particular to the systematic dither method, the moire problem is still left unsolved. Specifically, actual tone processing has to be preceded by an extra step of smoothing input data in order to remove the periodic components of dots. Such an extra step or preprocessing prevents the resolution of the system from being fully availed in the event of reproduction of a dot image. Even with error scattering method, the stripe pattern particular to two-level processing cannot be eliminated when a single output pixel is associated with single input pixel. Moreover, a false contour appears unless the multi-level output level is stable.